Microparticles are usually defined as particles 1 nm to 1 μm in size. Microparticles by themselves are used as stable monochromatic fluorescent particles or magnetic particles; in addition, they are used as building blocks of tunable light-emitting diodes, single-particle transistors, very-high-density magnetic recording media, and the like. In recent years, their fields of application have expanded along with technological advancement in individual fields, and this has led to an increase in demand.
There have been many reports on such microparticles including a metal, such as gold, platinum, or nickel, or a compound such as titanium oxide, zinc oxide, cadmium selenide, and zinc sulfide. As the production process therefor, a homogeneous precipitation method, a hydrothermal synthesis method, a hot-soap method, and the like have been known.
It has also been known that a porous oxide film having nanoholes periodically arranged into a triangular array at nanoscale intervals can be formed by an anodization process of aluminum (for example, nonpatent document 1). By carrying out a two-step anodization process, a periodic structure of nanoholes with improved regularity, perpendicularity, linearity, and independence can be formed (nonpatent document 2).
By using a method of filling nanoholes in an anodized film with a metal, semiconductor, or the like or a nanohole replica method, various applications have been attempted including coloring, magnetic recording media, EL light-emitting elements, electrochromic elements, optical elements, solar cells, gas sensors, and the like (patent documents 1 to 9). Furthermore, nanoholes are expected to be applied to various other fields including quantum devices such as quantum wires and MIM elements and molecular sensors with nanoholes used as chemical reaction sites (nonpatent document 3).    Nonpatent document 1: R. C. Furneaux, W. R. Rigby & A. P. Davids “NATURE” Vol. 337, p. 147 (1989)    Nonpatent document 2: Jpn. Journal of Applied Physics, Vol. 35, Part 2, No. 1B, pp., L 126-L 129, 15 January (1996)    Nonpatent document 3: Masuda “Kotai Butsuri [Solid Physics]” 31, 493 (1996)    Patent document 1: Japanese Unexamined Patent Application Publication No. 06-32675 (Patent No. 3004127)    Patent document 2: Japanese Unexamined Patent Application Publication No. 11-200090    Patent document 3: Japanese Unexamined Patent Application Publication No. 2000-285791    Patent document 4: Japanese Unexamined Patent Application Publication No. 2001-139317    Patent document 5: Japanese Unexamined Patent Application Publication No. 2001-166717    Patent document 6: Japanese Unexamined Patent Application Publication No. 2002-277659    Patent document 7: Japanese Unexamined Patent Application Publication No. 2003-073859    Patent document 8: Japanese Unexamined Patent Application Publication No. 2003-128832    Patent document 9: Japanese Unexamined Patent Application Publication No. 2004-130171